The present invention relates to a direct-injection internal combustion engine wherein a fuel is injected into a combustion chamber, and more particularly to techniques for stratifying an intake gas charge within the combustion chamber.
First Prior Art (JP-A-11-148429)
This prior art proposes a technique for stratifying an intake gas charge within a combustion chamber of a direct-injection type compression-ignition internal combustion engine provided with an exhaust gas recirculating (EGR) device, so that the amounts of harmful substances left in the exhaust gas are reduced by the stratification of the intake gas charge.
The combustion chamber is provided with two intake ports through which respective swirl flows are formed concentrically with each other in the same direction. The intake port located upstream of the swirl flows is provided to form the swirl flow of a small diameter in a central portion of the combustion chamber, while the intake port located downstream of the swirl flows is provided to form the swirl flow of a large diameter in a peripheral portion of the combustion chamber.
A recirculated exhaust gas is mixed with the intake gas flowing through the upstream intake port, but is not mixed with the intake gas flowing through the downstream intake port, so that the intake gas existing in a cylindrical region in the central portion of the combustion chamber contains the recirculated exhaust gas, while the intake gas existing in an annular region in the peripheral portion of the combustion chamber does not contain the recirculated exhaust gas.
This prior art says that the swirl flows of small and large diameters are formed in the respective radially inner and outer portions of the combustion chamber. The swirl flow, however, has a centrifugal force increasing its diameter. The swirl flow of the small diameter expands radially outwardly due to a centrifugal force thereof, with an increase of its diameter, and eventually collides and mixes with the swirl flow of the large diameter an increase of which is prevented by the cylindrical wall of the combustion chamber. Accordingly, it is difficult to stratify the intake gas charge within the combustion chamber such that the intake gas containing the recirculated exhaust gas exists in the central cylindrical region while the intake gas not containing the recirculated exhaust gas exists in the peripheral annular region.
Even if the swirl flows of small and large diameters are formed in the respective radially inner and outer portions of the combustion chamber in the intake stroke, there arises a squish flow of the intake gas from a squish area over a peripheral portion of the top face of the piston into a cavity in a central portion of the top face of the piston, in the subsequent compression stroke. The swirl flow of the large diameter in the peripheral portion of the combustion chamber is brought by the squish flow into the central portion of the combustion chamber, and collides and mixes with the swirl flow of the small diameter in the central portion of the combustion chamber. Therefore, it is difficult to maintain, up to a point of time near a terminal period of the compression stroke, the radially stratified state which has been established in the intake stroke and in which the intake gas charge consists of the inner cylindrical swirl flow and the outer annular swirl flow.
Thus, the present prior art arrangement does not assure that at a point of time near the terminal period of the compression stroke at which the combustion of the fuel injected into the combustion chamber is initiated, the central cylindrical region and the peripheral annular region of the combustion chamber are respectively charged with the intake gas containing the recirculated exhaust gas and the intake gas not containing the recirculated exhaust gas, or the intake gases having high and low concentration values of the recirculated exhaust gases, namely, the two intake gases of different compositions. It is not clear whether the amounts of harmful substances left in the exhaust gas are reduced.
The combustion of the fuel within the combustion chamber cannot be controlled as desired, unless the intake gas charge within the combustion chamber is stratified as needed, at the time of initiation of the fuel combustion.
Second Prior Art (FIG. 16)
A direct-injection type internal combustion engine of premixing type has an ordinary fuel injector 2 disposed in a center portion of the top surface of a combustion chamber 1, and a plurality of premixing fuel injector 31 which are disposed at respective positions in a peripheral portion of the top surface of the combustion chamber 1 and each of which is arranged to inject a fuel in a direction that is slightly inclined downwards with respect to the top surface of the combustion chamber. The plurality of premixing fuel injectors 31 are operated at a premixing fuel-injection timing before 30xc2x0 BTDC during the intake stroke or compression stroke, to inject a portion of a required amount of fuel. And the ordinary fuel injector 2 is operated to inject the remaining portion of the required amount of fuel, at a normal fuel-injection timing during the terminal period of the compression stroke.
This prior art makes it possible to reduce the amount of a portion of the fuel injected from the premixing fuel injectors 31 at the premixing fuel-injection timing, which portion adheres to the wall surfaces of the cavity of the combustion chamber 1. It is therefore possible to reduce the amounts of production of HC (hydrocarbon), SOF (soluble organic fraction) and white smoke due to quenching near the wall surfaces of the cavity.
Although the amount of HC and the like to be produced due to the adhesion of the premixing fuel to the wall surfaces of the cavity of the combustion chamber is reduced, the amount of HC and the like to be produced due to the fuel existing in the squish area of the combustion chamber is not reduced. Thus, the present prior art is not so effective to reduce the amount of HC and the like left in the exhaust gas.
The present prior art requires a plurality of premixing fuel injectors to be disposed in the peripheral portion of the top surface of the combustion chamber, in addition to the ordinary fuel injector disposed in the center portion of the top surface. Accordingly, the construction of the engine is complicated.
Third Prior Art (JP-B-2906932)
A lean-burn type spark-ignition internal combustion engine uses a plurality of intake ports through which two or three tumbling flows of intake gas are formed in the same direction, in respective right and left portions or respective right, left and central portions of the combustion chamber, during the intake stroke. A fuel is injected into only the intake port for forming the tumbling flow that passes a spark plug disposed on the top surface of the combustion chamber.
In this prior art, the tumbling flow of the intake gas containing the fuel does not pass the entirety of the squish area of the combustion chamber, but passes only a portion of the squish area, so that the amount of the fuel existing in the squish area of the combustion chamber is reduced, whereby the amount of production of HC and the like due to quenching in the squish area is reduced.
Although the fuel does not exist in a portion of the squish area that the tumbling flow of the intake gas not containing the fuel passes, the fuel exists in a portion of the squish area that the tumbling flow of the intake gas containing the fuel passes. In this respect, the present prior art is not effective enough to reduce the amount of the fuel existing in the squish area of the combustion chamber, and is therefore not effective enough to reduce the amount of production of HC and the like due to quenching in the squish area.
Study Relating to Stratification of Intake Gas Charge within Combustion Chamber
1) In a direct-injection type compression-ignition internal combustion engine, a jet of a fuel injected from the fuel injector into the intake air within the combustion chamber in the intake stroke causes an air surrounding a root portion, that is, the fuel injector side portion of the fuel jet, to be partly entrained within the interior of the fuel jet and partly carried with the exterior of the fuel jet, whereby an air flow accompanying the fuel jet is induced.
The fuel jet injected into the intake air within the combustion chamber flies at a high velocity, and breaks up into fine droplets, which are eventually vaporized and burned at the end portion of the fuel jet, to generate a flame. While the fuel jet may be burned to generate a flame at the root portion, the fuel jet is burned to generate a high-temperature flame on a large scale at the end portion.
While the fuel is injected and burned, there is formed in the combustion chamber a fuel-air-mixture forming region near the root portion of the fuel jet, in which a fuel-air mixture is formed by mixing the fuel and air. There is also formed in the combustion chamber a flame-generating region near the end portion of the fuel jet, in which the fuel is vigorously burned to generate a high-temperature flame on a large scale. Thus, the interior of the combustion chamber is roughly divided into the fuel-air-mixture forming region and the flame-generating region.
The composition of the fuel-air mixture formed in the fuel-air-mixture forming region of the combustion chamber is influenced by the composition of the intake gas existing in the fuel-air-mixture forming region upon injection of the fuel or upon initiation of the fuel combustion.
The fuel-air mixture formed in the fuel-air-mixture forming region of the combustion chamber is carried by the fuel jet or fuel-air mixture stream into the flame-generating region of the combustion chamber. In the flame-generating region, there exist the fuel-air mixture newly carried from the fuel-air-mixture forming region, and the intake air which has been present in the flame-generating region since a point before the initiation of the fuel combustion, and the burnt gas generated as a result of burning of the fuel in the flame-generating region. The combustion of the fuel in the flame-generating region occurs in the presence of those fuel-air mixture, intake air and burnt gas. The state of combustion of the fuel in the combustion chamber is influenced by the composition of the gas existing in the flame-generating region upon initiation of the fuel combustion.
In other words, the state of the fuel combustion within the combustion chamber can be controlled as desired, by controlling the composition of the gas existing in the fuel-air-mixture forming region upon injection of the fuel or upon initiation of the fuel combustion so that the composition is suitable for creating the desired fuel-air mixture, and by controlling the composition of the gas existing in the flame-generating region upon initiation of the fuel combustion so that the composition of this gas is suitable for establishing the desired fuel combustion state.
Namely, the fuel combustion state within the combustion chamber can be controlled by stratifying the intake gas charge within the combustion chamber such that the charge consists of the intake gas existing in the fuel-air-mixture forming region and the intake gas existing in the flame-generating region.
2) A distance from the nozzle holes of the fuel injector to a position at which the breakup of the fuel jet into fine droplets is initiated will be referred to as xe2x80x9ca spray breakup lengthxe2x80x9d of the fuel jet. The position at which the generation of a high-temperature flame on a large scale is initiated at the end portion of the fuel jet is spaced from the position of the nozzle holes of the fuel injector, by a distance which is 1-1.5 times the spray breakup length, which is equal to 15.8 (fuel density/air density)1/2xc2x7(diameter of nozzle hole of fuel injector).
The fuel injector has a multiplicity of nozzle holes disposed in a center portion of the top surface of the combustion chamber, which is opposed to the top face of the piston. The fuel is injected from these multiple nozzle holes in respective multiple radial directions, which are inclined relative to the nominal radial direction of the combustion chamber, toward the top face of the piston, so that the fuel is injected toward a peripheral part of a cavity formed in a center portion of the top face of the piston, at a point of time near the terminal period of the compression stroke.
Accordingly, the flame-generating region of the combustion chamber is an outside region and is spaced from the position of the nozzle holes of the fuel injector in each fuel injecting radial direction, by a distance not smaller than 1-1.5 times the spray breakup length of the fuel jet, and is substantially symmetrical with respect to the center axis of the combustion chamber. The fuel-air-mixture forming region is an inside region and is spaced from the position of the nozzle holes in each fuel injecting radial direction, by a distance not larger than 1-1.5 times the spray breakup length, and is substantially symmetrical with respect to the center axis of the combustion chamber.
It will be understood from the above analysis as follows. If the intake gas charge within the combustion chamber can be stratified, at a point of time near the terminal period of the compression stroke, so as to exist in two regions that are respectively located inside and outside a generally hemispherical or generally flat hemispherical plane which has its center at the center portion of the top surface of the combustion chamber from which the fuel is injected, the state of combustion of the fuel can be controlled by establishing the desired compositions of the two intake gases existing respectively in the inside and outside regions.
3) The intake gas charge within the combustion chamber can be stratified, in a direct-injection type compression-ignition internal combustion engine which has a plurality of intake ports provided to form a plurality of swirl flows of intake gases in the same direction in the combustion chamber, and which is arranged to inject a fuel from a center portion of the top surface of the combustion chamber, toward the peripheral part of a cavity formed in a center portion of the top face of the piston. The intake gas charge can be stratified by an intake stratifying device, which includes the configurations of the combustion chamber and the intake ports, which are suitably determined so as to establish desired dynamic characteristics of a squish flow and the swirl flows of the intake gases, as described below.
In the intake stroke, an intake port 3 located downstream of the swirl flows is arranged to form a swirl flow of a first intake gas 11 in an upper portion of the combustion chamber 1, along its cylindrical wall, while an intake port 4 located upstream of the swirl flows is arranged to form a swirl flow of a second intake gas 12 in a lower portion of the combustion chamber 1, along the cylindrical wall, as shown in FIG. 2. A state of the vertically stratified intake gas charge consisting of the swirl flows of the first and second intake gases 11, 12 of different compositions, as illustrated in FIGS. 4 and 5 by way of example, is maintained within the combustion chamber 1 up to a point of time within an intermediate period of the compression stroke.
In a latter half of the compression stroke in which a squish flow is created, the swirl flow on the peripheral portion of the top face of the piston is brought by the squish flow into the cavity in the central portion of the piston top face. In the presence of a centrifugal force produced by an increase of the velocity in the direction of the swirl flow with a decrease of the diameter of the swirl flow, the swirl flow is prevented from being directed to the central part of the cavity, but the swirl flow is caused to flow along the peripheral wall of the cavity and is directed toward the bottom surface of the cavity. Before the squish flow is created, the second intake gas exists in the entire portion of the cavity. After the squish flow is creased, the first intake gas 11 flows into the central region of the cavity, and the second intake gas 12 eventually exists in only the peripheral and bottom regions of the cavity, as indicated in FIGS. 6(a), 6(b) and 6(c) in the order of time elapse.
At a point of time near the terminal period of the compression stroke at which the combustion of the fuel is initiated, the first intake gas 11 primarily exists inside the generally flat hemispherical plane 13 which has its center at the center position of the top surface of the combustion chamber 1 from which the fuel is injected, while the second intake gas 12 primarily exists outside the generally flat hemispherical plane 13, as shown in FIG. 1. The intake gas charge within the combustion chamber 1 is stratified such that the intake gases 11, 12 of different compositions are inside and outside the generally flat hemispherical plane 13 at time of initiation of the fuel combustion.
3-1) When the recirculated exhaust gas is not mixed with the first intake gas 11 while the recirculated exhaust gas is mixed with the second intake gas 12, the intake gas charge within the combustion chamber 1 upon initiation of the fuel combustion is stratified such that the intake gas not containing the recirculated exhaust gas or having a low concentration of the recirculated exhaust gas exists in a region inside the generally hemispherical or generally flat hemispherical plane 13 having its center at the position of the fuel injection, while the intake gas containing the recirculated exhaust gas or having a high concentration of the recirculated exhaust gas exists in a region outside the generally hemispherical or generally flat hemispherical plane 13.
In the reverse case, that is, when the recirculated exhaust gas is mixed with the first intake gas 11 while the recirculated exhaust gas is not mixed with the second intake gas 12, the intake gas charge within the combustion chamber 1 upon initiation of the fuel combustion is stratified such that the intake gas containing the recirculated exhaust gas or having a high concentration of the recirculated exhaust gas exists in the region inside the generally hemispherical or generally flat hemispherical plane 13 having its center at the position of the fuel injection, while the intake gas not containing the recirculated exhaust gas or having a low concentration of the recirculated exhaust gas exists in the region outside the generally hemispherical or generally flat hemispherical plane 13.
3-2) The stratification pattern of the intake gas charge within the combustion chamber 1 is changed by mixing a specific component such as the recirculated exhaust gas and the fuel with the first intake gas 11 and the second intake gas 12, and by increasing or reducing the amounts of the specific component to be mixed with the first intake gas 11 and the second intake gas 12. Namely, the stratification pattern can be changed to one of: a normal stratification pattern in which the concentration of the specific component in the region of the combustion chamber 1 inside the generally flat hemispherical plane 13 is lower than that in the region outside the generally flat hemispherical plane 13; a reverse stratification pattern in which the concentration of the specific component in the region of the combustion chamber 1 inside the generally flat hemispherical plane 13 is higher than that in the region outside the generally flat hemispherical plane 13; and a homogeneous pattern in which the concentrations of the specific component in the regions of the combustion chamber 1 inside and outside the generally flat hemispherical plane 13 are equal to each other.
By increasing or reducing the amounts of the specific component to be mixed with the first intake gas 11 and the second intake gas 12, the degree of the stratification of the intake gas charge within the combustion chamber 1 can be changed. The degree of the stratification, that is, the ratio of the concentration of the specific component outside the generally flat hemispherical plane 13 to the concentration the specific component inside the generally flat hemispherical plane 13, is increased and reduced.
Study Relating to Stratification of Intake Gas Charge Containing Recirculated Exhaust Gas
4) According to a study by the present inventors on a direct-injection type compression-ignition internal combustion engine, a high-temperature region in which the fuel jet injected by the fuel injector is burned is formed in the squish area and the peripheral part of the cavity, as shown in FIGS. 11(a) and 12(a) by way of example.
In this high-temperature burning region, NOx (nitrogen oxides) is produced in a fuel-lean area in which the fuel-air ratio is in the neighborhood of the stoichiometric value, as shown in FIGS. 11(b), 11(c), 12(b) and 12(c) by way of example. When the squish area and the peripheral part of the cavity of the combustion chamber are charged with the intake gas containing the recirculated exhaust gas, during the fuel combustion period, the oxygen concentration and the combustion temperature in the high-temperature burning region are lowered, with a result of reduction of the amount of production of NOx.
As shown in FIGS. 11(b), 11(d), 12(b) and 12(d) by way of example, soot is produced in a fuel-rich area or oxygen-lean burning area of the high-temperature burning region, in which the fuel-air ratio is lower than the stoichiometric value, that is, in the end portion of the jet of the fuel injected by the fuel injector, or in the recessed portion of the cavity. To reduce the amount of production of the soot, it is effective to supply oxygen to the end portion of the fuel jet in which the fuel combustion takes place with a shortage of oxygen, that is, to eliminate the shortage of oxygen in the end of the fuel jet. When a circumference of the root portion of the fuel jet injected from the fuel injector is not supplied with the recirculated exhaust gas but is supplied with fresh air during the fuel combustion period, the fresh air is carried or fed by the fuel jet or fuel-air mixture stream to the end portion of the fuel jet or the recessed portion of the cavity in which the fuel combustion takes place with a shortage of oxygen, so that the amount of production of the soot is reduced. At this time, however, an increase of NOx is prevented by controlling the oxygen concentration so as to prevent its increase to a level of production of NOx.
Accordingly, the amounts of production of NOx and soot can be both reduced by stratifying the intake gas charge within the combustion chamber upon initiation of the fuel combustion such that the intake gas not containing the recirculated exhaust gas or having a low concentration of the recirculated exhaust gas exists in the fuel-air-mixture forming region including the root portion of the fuel jet injected from the fuel injector, while the intake gas containing the recirculated exhaust gas or having a high concentration of the recirculated exhaust gas exists in the flame-generating region located outside the fuel-air-mixture forming region.
5) When the internal combustion engine is operated under a high load and the timing of the fuel injection is late, or when the engine is operated at a high speed and a strong reverse squish flow is created, the amount of the fuel flowing out of the cavity in the combustion chamber tends to increase, so that the fuel combustion outside the cavity takes place with a shortage of oxygen, while the fuel combustion within the cavity takes place with an excessive amount of oxygen. In this case, the soot is produced primarily outside the cavity, while the NOx is produced primarily within the cavity.
In the above case, the normal stratification pattern within the combustion chamber is changed to the reverse stratification pattern in which the intake gas containing the recirculated exhaust gas or having a high concentration of the recirculated exhaust gas exists in the central portion of the combustion chamber including the position of the fuel injection, while the intake gas not containing the recirculated exhaust gas or having a low concentration of the recirculated exhaust gas exists in the outer peripheral portion of the combustion chamber. In the reverse stratification pattern, the oxygen concentration is increased outside the cavity and in the squish area of the combustion chamber, so that not only the amount of production of the soot is reduced, but also oxidization of the soot is promoted, reducing the amount of the soot. At this time, however, an increase of NOx is prevented by controlling the oxygen concentration so as to prevent its increase to a level of production of NOx.
The amounts of production of NOx and soot can be effectively reduced by controlling the stratification pattern of the intake gas charge within the combustion chamber 1, depending upon the operating conditions of the internal combustion engine.
6) When the intake gas containing the recirculated exhaust gas or a high concentration of the recirculated exhaust gas exists in the region outside the generally hemispherical or generally flat hemispherical plane having its center at the position of the fuel injection, the radius of the generally hemispherical or generally flat hemispherical plane increases with an increase in the concentration of the recirculated exhaust gas outside the generally hemispherical or generally flat hemispherical plane, where the amount of the recirculated exhaust gas contained in the intake gas is held constant.
When the area of production of NOx is wide enough to cover a part of the region inside the generally hemispherical or generally flat hemispherical plane, the amount of production of NOx can reduced over the wide area, by reducing the concentration of the recirculated exhaust gas outside the generally hemispherical or generally flat hemispherical plane, to thereby reduce the radius of the generally hemispherical or generally flat hemispherical plane. When the area of production of NOx is narrow, on the other hand, the amount of production of NOx can be reduced with an increase in the concentration of the recirculated exhaust gas, by increasing the concentration outside the generally hemispherical or generally flat hemispherical plane, to thereby increase the radius of the generally hemispherical or generally flat hemispherical plane.
Thus, the amounts of production of NOx and soot can be more effectively reduced by changing the concentration of the recirculated exhaust gas inside or outside the generally hemispherical or generally flat hemispherical plane, depending upon the area of production of NOx, that is, the operating conditions of the internal combustion engine.
Study Relating to Stratification of Intake Gas Charge Containing Fuel
7) In the internal combustion engines according to the second and third prior arts, the amounts of harmful substances such as HC, SOF and white smoke left in the exhaust gas cannot be sufficiently reduced, in the presence of the fuel in the squish area and near the wall surfaces of the cavity of the combustion chamber, which tend to easily suffer from a temperature drop and quenching. It was found effective to stratify the intake gas charge within the combustion chamber upon initiation of the fuel combustion such that the intake gas not containing the fuel or having a low concentration of the fuel gas exists in the squish area and near the wall surfaces of the cavity in the peripheral portion of the combustion chamber, while the intake gas containing the fuel or having a high concentration of the fuel exists in the central portion of the combustion chamber.
The thus stratified intake gas charge prevents the presence of the fuel, or reduces the amount of the fuel, in the squish area and near the wall surfaces of the cavity of the combustion chamber, upon initiation of the fuel combustion, so that the amount of the fuel present in the region which tends to easily suffer from quenching is reduced. Further, the above-indicated stratification permits the fuel combustion primarily in the central portion of the combustion chamber, making it possible to increase the fuel combustion temperature and consequently reduce the non-combustion ratio of the fuel. As a result, the amounts of the harmful substances such as HC, SOF and white smoke left in the exhaust gas are more effectively reduced.
8) An internal combustion engine has a characteristic that the temperatures in the central and peripheral portions of the combustion chamber are both higher when the load on the internal combustion engine is high than when the load is low. When the load is high, a ratio of the fuel mixing ratio of the intake gas existing upon initiation of the fuel combustion in the peripheral portion of the combustion chamber to that of the intake gas existing in the central portion of the combustion chamber, which central portion includes the center portion of the top surface of the combustion chamber, is made higher than when the load is low. As a result, the fuel concentration in the central portion of the combustion chamber is reduced, preventing an excessive rise of the combustion temperature and thereby reducing the amount of increase of NOx. In the peripheral portion of the combustion chamber, on the other hand, the fuel concentration is high, but an increase of the amount of HC is prevented owing to a high temperature in the peripheral portion. Rather, a ratio of the fuel to the oxygen is increased in the peripheral portion, so that the oxygen can be effectively utilized.
The amounts of the harmful substances left in the exhaust gas can be more effectively reduced by controlling the ratio of the fuel concentration of the intake gas existing upon initiation of the fuel combustion in the peripheral portion of the combustion chamber to that in the central portion of the combustion chamber including the center portion of the top surface of the combustion chamber, depending upon the operating conditions of the internal combustion engine.
Stratification of Intake Gas Charge within Combustion Chamber
1) A method of stratifying an intake gas charge within a combustion chamber of a direct-injection type internal combustion engine wherein a fuel is injected into the combustion chamber, wherein the intake gas charge is stratified such that intake gases of different compositions exist in a central portion of the combustion chamber including a position of injection of the fuel, and in a peripheral portion of the combustion chamber, upon initiation of combustion of the fuel at a point of time near a terminal period of a compression stroke.
The xe2x80x9cintake gases of different compositionsxe2x80x9d may be xe2x80x9cintake gases having different concentrations of a specific component contained therein such as a recirculated gas and a fuelxe2x80x9d, for instance.
2) A method according to the above stratifying method, wherein a pattern of stratification of the intake gas charge within the combustion chamber is changed depending upon operating conditions of the internal combustion engine, to a selected one of: a normal stratification pattern in which the concentration of the specific component of the intake gas in the central portion is lower than that of the intake gas in the peripheral portion; a reverse stratification pattern in which the concentration of the specific component of the intake gas in the central portion is higher than that of the intake gas in the peripheral portion; and a homogeneous pattern in which the concentration of the specific component of the intake gas in the central portion is equal to that of the intake gas in the peripheral portion.
3) A method according to the above stratifying method, wherein a degree of the stratification, that is, a ratio of the concentration of the specific component of the intake gas in the peripheral portion to that of the intake gas in the central portion is changed depending upon operating conditions of the internal combustion engine.
4) A device for stratifying an intake gas charge within a combustion chamber of a direct-injection type internal combustion engine in which a plurality of intake ports are provided to form a plurality of swirl flows of intake gases in a same direction in the combustion chamber and in which a fuel is injected into the combustion chamber, from a center portion of a top surface of the combustion chamber opposed to a top face of a piston, toward a peripheral part of a cavity formed in a central portion of the top face of the piston, the device including:
an arrangement for forming, in an intake stroke, a swirl flow of a first intake gas in an upper portion of the combustion chamber, along a cylindrical wall of the combustion chamber, and a swirl flow of a second intake gas in a lower portion of the combustion chamber, along the cylindrical wall of the combustion chamber, and for maintaining a state of vertical stratification of the intake gas charge consisting of the swirl flows of the first and second intake gases within the combustion chamber, up to a point of time within an intermediate period of a compression stroke;
an arrangement for causing the first intake gas to flow into a central region of the cavity in the central portion of the top face of the piston, while the second intake gas remains in a peripheral region and a bottom region of the cavity, in a latter half of a compression stroke in which a squish flow is created; and
an arrangement for stratifying the intake gas charge within the combustion chamber at a point of time near a terminal period of the compression stroke in which the combustion of the fuel is initiated, such that the first intake gas exists primarily in a region inside a generally hemispherical or generally flat hemispherical plane having its center at a position of injection of the fuel into the combustion chamber, while the second intake gas exists primarily in a region outside the generally hemispherical or generally flat hemispherical plane.
5) A device according to the above stratifying device, wherein the generally hemispherical or generally flat hemispherical plane is spaced from the position of injection of the fuel into the combustion chamber, in a direction of injection of the fuel, by a distance 1-1.5 times a spray breakup length of the injected fuel.
Stratification of Intake Gas Charge Containing Recirculated Exhaust Gas
6) A direct-injection type internal combustion engine wherein a fuel is injected into a combustion chamber, and an intake gas not containing a recirculated exhaust gas or having a low concentration of the recirculated exhaust gas exists in one of a central portion of the combustion chamber including a position of injection of the fuel and a peripheral portion of the combustion chamber, while an intake gas containing the recirculated exhaust gas or having a high concentration of the recirculated exhaust gas exists in the other of the central and peripheral portions of the combustion chamber, upon initiation of combustion of the fuel at a point of time near a terminal period of a compression stroke.
7) A direct-injection type internal combustion engine according to the above internal combustion engine, wherein the intake gas containing the recirculated exhaust gas or having the high concentration of the recirculated exhaust gas exists in the central portion of the combustion chamber, while the intake gas not containing or having the low concentration of the recirculated exhaust gas exists in the peripheral portion, upon initiation of combustion of the fuel at the point of time near the terminal period of the compression stroke, when the internal combustion engine is operated under a high load or at a high speed.
8) A direct-injection type internal combustion engine according to the above internal combustion engine, wherein a ratio of the concentration of the recirculated exhaust gas in the peripheral portion to the concentration of the recirculated exhaust gas in the central portion is changed depending upon operating conditions of the internal combustion engine.
Stratification of Intake Gas Charge Containing Fuel
9) A direct-injection type internal combustion engine of compression-ignition or spark-ignition type in which a fuel is injected into an intake gas within a combustion chamber or to the intake gas within the combustion chamber and an intake gas within an intake passage and in which combustion of the fuel is initiated at a point of time near a terminal period of a compression stroke, the direct-injection type internal combustion engine including:
an arrangement for stratifying an intake gas charge within the combustion chamber such that an intake gas containing a fuel or having a high concentration of the fuel exists in a central portion of the combustion chamber including a center portion of a top surface of the combustion chamber, while an intake gas not containing the fuel or having a low concentration of the fuel exists in a squish area and near wall surfaces of a cavity in a peripheral portion of the combustion chamber, upon initiation of combustion of the fuel.
10) A direct-injection type internal combustion engine according to the above internal combustion engine, wherein a ratio of the concentration of the fuel in the intake gas existing upon initiation of combustion of the fuel in the peripheral portion of the combustion chamber, to the concentration of the fuel in the intake gas existing in the central portion of the combustion chamber including the center portion of the top surface is changed depending upon operating conditions of the internal combustion engine.